Enable DAHDI support by default; require --disable-dahdi otherwise

libosmo-abis was built with DAHDI support, if the related header files
were present at built time, and without if not.  This kind of automagic
enabling/disabling of features is wrong.  Let's require DAHDI support by
default, and force the user to take a conscious decision by using an
explicit --disable-dahdi if he doesn't want it.

At the same time, update debian/control to list dahdi-source as build
dependency.

Change-Id: Id9f7f063e7ca9e3ab4aa96fc93f243caf50fb66a
Closes: OS#4248
diff --git a/src/Makefile.am b/src/Makefile.am
index f4277ae..9c438c9 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -19,7 +19,6 @@
 			 ipa_proxy.c \
 			 subchan_demux.c \
 			 trau_frame.c \
-			 input/dahdi.c \
 			 input/ipa.c \
 			 input/ipa_keepalive.c \
 			 input/ipaccess.c \
@@ -28,6 +27,9 @@
 			 input/misdn.c \
 			 input/rs232.c \
 			 input/unixsocket.c
+if ENABLE_DAHDI
+libosmoabis_la_SOURCES += input/dahdi.c
+endif
 
 libosmotrau_la_CFLAGS = $(AM_CFLAGS) $(ORTP_CFLAGS)
 libosmotrau_la_LDFLAGS = $(AM_LDFLAGS) -version-info $(TRAU_LIBVERSION)